Photographic bleach bath



United Sttes Patent PHOTOGRAPHIC BLEACH BATH Harris D. Hineline, Mount Vernon, N. Y., assignor to General Aniline & Film Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application August 7, 1952, Serial No. 303,161

3 Claims. (Cl. 96-60) This invention relates to a photographic reversal process, to an improved bleach bath for said process, and particularly to an improved process of bleaching.

It is known that in the photographic process, called in the art as the reversal process, in which positive photographic images are made upon the same transparent base and in the same light-sensitive silver halide emulsion, as the originally exposed negative image, it is necessary to remove the negative silver image (produced by exposure and a first development) by converting it into a soluble silver salt by means of a suitable bleach bath. Various bleach bath formulae, such as aqueous solutions of potassium dichromate and sulfuric acid, potassium ferricyanide and potassium permanganate, potassium permanganate and sulfuric acid or other acid, and the like, with or without other adjuvants, have been proposed for effectuating the bleaching treatment, i. e., converting the metallic silver into soluble salt.

All of these baths possess certain undesirable characteristics. The bleach bath consisting of an aqueous solution of potassium dichromate and sulfuric acid imparts a residual, yellow, highlight stain on the final positive image, particularly when the first development is conducted in a non-caustic developer. The bleach bath consisting of an aqueous solution of potassium permanganate and mineral acid has a tendency to lower the physical strength of the emulsion layer by causing reticulation, and in some cases a very serious deterioration of the gelatin resulting and excessive softening and frilling of the gelatin layer.

An object of the present invention is to provide an improved bleach bath for the photographic reversal process.

A further object is a bleach bath which removes a negative silver image While avoiding staining.

A still further object is to provide an improved bleaching process whereby a developable silver halidepositive image of finer grain and better contrast in a tougher gelatin matrix is obtained.

Other objects and advantages of the invention will become apparent from the following disclosure.

I have found that the above objects are accomplished by employing as a bleach bath (after the first development) an aqueous solution containing from 2 percent to 25 percent of an organic per-oxy-acid having from 1 to 9 carbon atoms per molecule as such, or in admixture with 0.5 percent to percent of potassium permanganate or 3 percent to 5 percent of potassium dichromate in acid solution. This discovery makes it possible to remove a negative silver image while avoiding frilling, reticulation, and staining of the gelatin. When employing the per-oxy-acid alone, the concentration may range from 20 to 200 grams per liter of water. On the other hand, when the bleach bath contains either potassium dichromate or potassium permanganate, the concentration of the per-oxy-acid may range from to 100 grams per' liter. Instead of employing the acid solution containing potassium permanganate or potassium dichromate,

3 to 5 grams of potassium dichromate and 0.5 to 5 grams of potassium permanganate may be employed in admixture with the organic per acid to yield a non-staining bleach bath.

When employing the organic per-oxy-acids in aqueous solution, their action is to dissolve the reduced silver without harm to the gelatin matrix or with a very valuable tanning action, and at the same time modify the residual silver halide to yield a much finer grain image, a warmer color, and a most unexpected and valuable resistance to overexposure.

As examples of suitable per-oxy-acids which may be employed as such or in admixture with either potassium permanganate, potassium dichromate, or a mixture of the latter two, the following may be given:

Per-oxy-formic acid Per-oxy-acetic acid Per-oxy-propionic acid Per-oxy-butyric acid Per-oxy-oxalic acid Per-oxy-citric acid Per-oxy-succinic acid Per-oxy-furoic acid Per-oxy-benzoic acid There listed acids are given only as representatives of a considerable range of analogous acids and of the several homologous series of such organic per-oxy acids.

In the case of per-oxy-benzoic acid which is only slightly soluble in water, it is advisable to use either alcohol or acetone as a solvent with the per-oxy-acid and then use such solution diluted with water. In the case where a mixtures of the per-oxy-acids, potassium dichromate, potassium permanganate or a mixture of the latter two are employed, the same procedure may be used, i. e., the slightly water soluble per-oxy-acid may be first dissolved in either acetone or alcohol, and the solution incorporated into the water solution containing the potassium dichromate, potassium permanganate, or a mixture of the latter two. I prefer, however, to use per-acetic acid because of its commercial availability and ease of handling.

In order to remove any traces of the bleach bath con taining the per-oxy-acid, it is desirable with some of the per-oxy-acids that the per-oxy-acid retained in the emulsion be reduced by any convenient reducing agent, such as sodium sulfite, etc. With some of the per-oxy-acids however this is not necessary, since they can be washed out by water alone.

In practicing the present invention, the first item is the sensitive material. It does not, however, appear that the film or plate used is at all critical, although, of course, an emulsion particularly designed for reversal is preferable. The photographic material may be exposed in the usual way and developed in any convenient first developer. The well known metol hydroquinone formula, sold as D 76 is a convenient fine grain developer for the first development.

This developer is made up according to the following recipe:

Water (at about F.) ounces 64 Metol (p-amido phenol hydrochloride) grains 116 Sodium sulphite ounces 13 /2 Hydroquinone grains 290 Kodalk (borax) -do 116 Water to make gall-on.

ganic per acid. Five to ten minutes in running water is usually sufficient.

The next step is the essence of the present invention. For this step, the developed material is placed in the peracid-solution. If per acetic acid is used (this being, the preferred acid) the solution should contain from two to five percent of the acid in water. This strength of acid will give complete solution of the silver negative image in about five minutes, or a little more. The solution reaction with the per-acid appears to be wholly different from that of any of the previous solvents because of the fact that a substantial amount of oxygen is liberated. It will be remembered that silver is a mono-valent metal whereas oxygen is bivalent. It appears that the silver must be oxidized before it can be dissolved by the acid, which is done by the per oxygen, but there seems to be twice as much oxygen in the per-oxy-acetic acid as is needed to oxidize the amount of silver which can be combined with the acid to yield the soluble silver salt. Hence, the excess is liberated as molecular oxygen gas, which must diffuse through the gelatin. If five minutes or so are allowed for this diffusion, it takes place smoothly and easily. If, however, it is attempted to speed up the procedure, gas bubbles form in the emulsion and may cause damage, especially to old or tender emulsions.

Also, in the case of per-oxy-acetic acid, it is desirable that the acid be used in a rather low concentration for another reason. The per-oxy-acetic acid is quite pungent, more so than acetic acid as such, and the fumes are slightly irritating. Accordingly, the lower the concentration the less troublesome the fumes. However, the fumes are very heavy and the simple procedure of covering the tray or tank with a loose lid subdues them so that the odor practically completely disappears from the solutions at a concentration below seven to nine percent.

When the metallic silver has been completely dissolved it will be noticed that the surface of the emulsion has a very hard, leathery feel, and a somewhat greasy appearance. This seems to be due to an actual combination between the gelatin and the per-oxy-acetic acid, since no amount of washing in water will remove it. Perhaps it is some sort of a condensation or polymerization reaction. At any rate, if an attempt is made to develop the residual silver halide to a positive image, directly after treatment with the per-oxy-acetic acid it is found that the silver image is wholly lost in a very dense, heavy stain deposit which seems to be nearly uniform over the entire picture area. Accordingly, a treatment is required to remove this retained per acid.

For this step, the emulsion, on its support, is desirably rinsed, briefly in water, to remove any loose acid, and then submerged in a reducing agent. This agent preferably is sodium sulphite or bisulphite of a mixture of the two in Water solution. The concentration is not critical, but a convenient concentration is about ten percent. If the strength is too low the rate of reduction of the per acid residue is unduly slow, and if the concentration is too high the raised osmotic pressure slows down the rate of diffusion of the sulphite into the gelatine unduly, again slowing the clearing action. This treatment removes all of the oiliness present after the per acid treatment and removes much, but not all, of the hard tanning produced by the per acid, leaving the emulsion ready, after a brief rinsing, for the second development, which of course is preceded by an exposure to light.

For this second development, any desired or convenient developer may be used. The same D 76 deevloper used for the first development may be used, .to yield a very fine grained positive image. It may be noticed that this positive image has a much finer grain than the negative image had, due seemingly to some effect of the per acid on the emulsion, perhaps aided by the subsequent reducer.

If a controlled flash exposure has been used before the second development, there may be undeveloped silver halide left in the image and if so this should be removed by fixing in any standard fixing solution. If the residual silver halide has been fully exposed and developed there will be little or no residual silver halide left and fixing is then unnecessary. It may however be used as a precautionary measured if desired.

The finished positive may then be washed and dried as usual, whereupon it is ready for the usual purposes of a photograph.

It may be noticed also that for the silver solvent, it is necessary that the per oxygen be combined in the molecule; a mixture of acetic acid and hydrogen peroxide is wholly without effect as a silver solvent.

Example I Two sheets of standard portrait film were exposed, one for l second at f 4.5 as indicated by the meter, the other for second at f 4.5. Both were then developed in D 76, as shown in the above recipe, at normal strength for 30 minutes. The developer was then rinsed out by about ten minutes treatment in running water and then placed in the per-acid solvent solution. This was prepared by mixing three ounces of the 40 percent peracetic acid as purchased with seventeen ounces of water. The dissolving step required about six minutes, after which the films were returned to the wash water for a short rinse and then placed in a solution of sulphite and bisulphite containing about an ounce of each in twenty ounces of water. About ten minutes soaking cleared out the residual per-acid; whereupon the films weer again rinsed briefly, to remove the excess sulphite, and returned to the developer for twenty-five minutes. They were then given a short treatment in an acid fix as a precautionary measure and washed. The result was two very good positives of surprisingly little difference in density.

Example I] A length of a standard 16 mm. positive stock was exposed in a camera at a scene which the meter indicated should have a lens opening of about f 6.3. However, a series of different exposures were given in the sequence f 1.5, 1.9, 2.7, 3.5, 4.5, 6.3, 8, 11, and 16. The resulting strip of film was then processed as in Example I. A very high contrast was obtained; the 1.5 exposure was undesirably thin; but the others up to and including the f 8 exposure were of increasing density, but fully satisfactory; the 4.5 being perhaps the best.

Example 111 A strip of blue base 16 mm. negative film was loaded into the camera and exposed in the same series of lens openings as in Example II, but under light conditions which were considerably darker so that the meter indicated an exposure of about 4.5 as correct. This was first developed as in Example I, reversed, redeveloped, etc. This film apparently had more silver in the emulsion, or a thicker emulsion, since only the exposures from f 1.5 to 4.5 were usable, and the best was the 3.5 exposure.

Example IV A sheet of standard portrait film was given a liberal exposure, as in Example I, developed in D 76 as before, rinsed briefly and treated in a saturated solution (exact concentration unknown, but probably about seven percent) of per-oxy-succinic acid. Solution of the silver was a little slower than with the per-acetic acid, but complete in about ten minutes. The per-oxy-succinic acid seems to show much less condensing effect with the gelatin. There was none of the hardening effect nor the surface oiliness characteristic of the per-acetic acid. When solution of the first developed silver was complete, the film was rinsed in water, given a short dip into the same sulphite-bisulphite bath (although it hardly seemed necessary) given the needed second exposure and then the second development, followed by a short fixing, washing and drying, to yield a very good positive film.

Examp le V A sheet of airplane color film containing the color formers in the film (war surplus stock) was exposed strictly according to the meter reading and developed in the supplied first developer. Then the developer was) washed out with some care, the film placed in the peroxy-acetic bath of Example I and the light turned up. Solution of the silver was complete in less than ten minutes and the film was then rinsed and treated in the same sulphite-bisulphite as in Example I for about ten minutes. After this treatment the film was again rinsed, to remove the sulphites and avoid any upset in the second developer composition, and second developed in the color developer as supplied. The second, color, developer was then rinsed out with some care, to avoid stain, and the film returned to the per-acetic acid bath until the silver of the second development was dissolved out. This treatment gave a distinct browning of the colors, but upon treatment with the mixed sulphites as before, the colors were restored to the proper brilliance. It may be noted that by dissolving the silver in two steps the second exposure is much shortened and the process can be watched throughout, making it much easier for the ordinary darkroom worker to conduct it.

Example VI Three strips of 16 mm. conventional reversal film of l-foot lengths were identically exposed and developed for 1-0 minutes at 70 F. in D-76 of the composition above given and then washed for 2 /2 to 5 minutes in running water. The first film strip was then bleached in a bleach solution containing 5 grams of potassium permanganate, 4 cc. of 98 percent sulfuric acid, and water to make up 1 liter. The second film strip was bleached in a solution containing 5 grams of potassium dichromate and 5 cc. of 98 percent sulfuric acid with sutficient water added to make up 1 liter. The third film strip was bleached in a solution containing 3 grams of potassium permanganate and 15 cc. of 40 percent aqueous per-acetic acid. The three bleached film strips were again washed with water and immersed in a clearing bath containing 35 grams of anhydrous sodium sulfite and sufiicient water to make 1 liter. After rinsing in water for 5 to lOminutes, each film was re-exposed and developed for to minutes at 68 F. in a developer of the following composition:

Water to make up 1 liter.

The three developed film strips were washed five minutes in water, fixed for 5 minutes in a conventional sodium thiosulfate fixing bath, and washed for 10 minutes. After normal drying for 10 minutes, the film' strip bleached in the bleach bath containing potassium permanganate and sulfuric acid was very soft and the film strip bleached in the bleach solution containing potassium permanganate and sulfuric acid showed a residual yellow highlight stain while the film strip bleached in potassium permanganate containing per-acetic acid showed clean highlights with no evidence of frilling or other physical deterioration.

The above examples show four types of film and two solvents. Other tests indicate that any of the usual gelatino-silver emulsions will work equally well. Also, it appears from other tests that this reaction is a general one and that any organic per acid will dissolve the silver, provided only that the solubility in water is at least as high as about 3 percent and that the-resulting silver salt is soluble in an amount at least as high as 0.25 percent. It may be noted that there isan interesting difference between the two acids shown; in that the per acetic acid is a monovalent acid with one atom of per-oxygen, whereas the per-oxy-succinic acid is a divalent acid with an atom of per-oxygen. In consequence, the per acetic acid has an excess of oxygen, whereas the per-succinic acid does not.

While there are above disclosed but a limited number of embodiments of the invention here presented, it is possible to produce still other embodiments thereof without departing from the inventive concept herein disclosed, and it is therefore desired that only such limitations be imposed on the appended claims as are stated therein.

The invention claimed is:

l. A photographic bleach bath comprising an aqueous solution containing a member selected from the class consisting of potassium dichromate and potassium permanganate, and a per-oxy-acid, having from 1 to 9 carbon atoms in admixture therewith.

2. A photographic bleach bath consisting of an aqueous solution containing 3 to 5 grams of potassium dichromate 0.5 to 2 grams of potassium permangate and 10 to 15 cc. of 40 percent per-acetic acid.

3. A photographic bleach bath consisting of an aqueous solution containing 3 to 5 grams of potassium dichromate 0.5 to 2 grams of potassium permanganate and 10 to 15 cc. of per-oxy-succinic acid.

References Cited in the file of this patent Bennett: Concise Chemical and Technical Dictionary," Chemical Publ. 00., Brooklyn, New York, 1947, page 709. Copy in Div. 67. 

1. A PHOTOGRAPHIC BLEACH BATH COMPRISING AND AQUEOUS SOLUTION CONTAINING A MEMBER SELECTED FROM THE CLASS CONSISTING OF POTASSIUM DICHROMATE AND POTASSIUM PERMANGANATE, AND A PER-OXY-ACID, HAVING FEOM 1 TO 9 CARBON ATOMS IN ADMIXING THEREWITH. 